1. Field of the Invention
This invention relates to moisture curable polyisobutylenes. More particularly, this invention relates to moisture curable organosiloxy-terminated isobutylene oligomers wherein the terminal positions of the molecules are occupied by a silicon atom bonded to three hydrolyzable groups. The polymers combine the excellent barrier properties of polyisobutylene with the curing properties of moisture curable silicone elastomer compositions.
2. Description of the Prior Art
The class of organic rubbers referred to as "butyl rubber" is superior to other organic rubbers and many silicone rubbers because of its low permeability to gases, including water vapor, high coefficient of friction and resistance to hydrolytic degradation. Butyl rubbers are copolymers wherein about 98 mole percent of the repeating units are derived from isobutylene and the remaining repeating units are derived from isoprene.
Commercially available butyl rubber is typically in the form of a high viscosity gum that is cured by reacting it with organic peroxides or sulfur in the presence of heat. The consistency of the curable compositions and the requirement for heating to achieve curing makes it impractical to utilize the excellent barrier properties of cured butyl rubber in the form of films or coatings exhibiting thicknesses of less than 1 mm.
U.S. Pat. No. 4,342,849, which issued to Kennedy on Aug. 3. 1982 discloses the preparation of telechelic or terminally functional linear polyisobutylenes exhibiting molecular weights of from 500 to 50,000. The lower molecular weight polymers are liquids at 25.degree. C. One class of polymers described in this patent contain carbon-to-carbon double bonds at the terminal positions. The inventor suggests that these polymers can be hydrosilylated to yield "a great variety of other products", but does not disclose a method or suitable organosilicon compounds for carrying out the hydrosilylation.
Curing of liquid isobutylene oligomers by reacting them with organohydrogensiloxanes is reported in an article by the present inventor and C. W. Macosko that appeared in Polymer Preprints (volume 26, no. 2, page 48). published during August of 1985. Oligomers containing from 18 to 20 repeating units per molecule were reacted with either ##STR3## at elevated temperatures in the presence of cis-(Et.sub.2 S).sub.2 PtCl.sub.2 as a hydrosilation catalyst. In these formulae Me represents methyl and Et represents ethyl. The reaction product with A was a clear polymer that dissolved in organic solvents, while the reaction product with B was a gel or a dry elastomer, both of which did not dissolve in organic solvents. In each instance nuclear magnetic resonance data indicated formation of both the expected hydrosilation reaction product and substantial amounts of undesired gaseous by-products that were postulated to be silanes.
It is also known to react organic polymers containing at least two sites of ethylenic unsaturation per molecule with organosilicon compounds containing a silicon-bonded hydrogen atom and at least two hydrolyzable groups such as alkoxy or carboxyl. The resultant products cure at room temperature in the presence of atmospheric moisture.
U.S. Pat. No. 4,435,536, which issued to Kato et al. on Mar. 6, 1984 teaches reacting an organic resin containing pendant and/or terminal vinyl radicals with a silane wherein the silicon atom is bonded to a hydrogen atom and at least one alkoxy group. The resultant resin is curable in the presence of moisture. The types of vinyl resins that can be modified in this manner include styrene and alpha-methylstyrene. The organosilicon compounds disclosed as being useful for reacting with these organic resins are silanes containing a silicon-bonded hydrogen atom and at least two hydrolyzable groups such as alkoxy that are bonded to the silicon atom by means of an oxygen or a nitrogen atom.
In an article appearing in Macromolecular Chemistry (page 1279 of volume 186. published in 1985) Yamashita and coworkers report the synthesis and curing of high molecular weight alkoxysilyl functional butyl rubbers. The curable material was prepared by the reaction of aminopropyltriethoxysilane with commercially available halogenated butyl rubber that had previously been reacted to incorporate a secondary amine or an amine hydrohalide salt structure at the crosslink sites. Complete curing of this material could be achieved only by direct contact with liquid water at a temperature of 70.degree. C. for at least 24 hours. The high viscosity of this composition and the requirement for heating it in contact with liquid water to achieve curing make the composition unsuitable for use as a room temperature vulcanizable composition.
It would be desirable to eliminate both undesirable side reactions during preparation of a curable liquid polyisobutylene and the heating required to achieve curing of prior art alkoxysilyl functional polymers by providing a liquid polyisobutylene containing terminal units of the formula --SiX.sub.3 where X represents a hydrolyzable group such as alkoxy or acyloxy.
U.S. Pat. No. 4,524,187, which issued to Greco et al. on June 18, 1985 discloses reacting isobutylene/triene copolymers containing pendant vinyl radicals with a silicon hydride of the general formula HSiR.sub.a X.sub.3 --.sub.a. where R represents a monovalent hydrocarbon radical containing from 1 to 10 carbon atoms, X represents a halogen atom or a hydrolyzable group such as alkoxy, phenoxy, acyloxy, amino, aminoxy or oxime and the value of a is 1, 2 or 3. The reaction products, some of which are liquid at 25.degree. C., cure in the presence of atmospheric moisture due to the presence of the hydrolyzable X groups bonded to the silicon atoms.
The only silicon hydride exemplified in the aforementioned Greco et al. patent is trichlorosilane. The hydrosilation reaction product of this compound with a isobutylene/triene copolymer contains silicon-bonded halogen atoms. This reaction product is unsuitable for use as a room temperature vulcanizable composition because of the toxic and highly corrosive hydrogen chloride generated as a by-product of the curing reaction. Room temperature vulcanizable organosiloxane compositions typically contain alkoxy, acetoxy or ketoximo groups as the silicon-bonded hydrolyzable group. These groups produce considerably less objectionable by-products during curing of the polymer.
Attempts by the present inventor to introduce trialkoxysilyl groups into a linear liquid polyisobutylene containing a CH.dbd.C(CH.sub.3)- radical at each of the two terminal positions by reacting the polymer with triethoxysilane, HSi(OC.sub.2 H.sub.5).sub.3, in the presence of a platinum-containing hydrosilation catalyst were unsuccessful. This silane is disclosed in the aforementioned U.S. Pat. No. 4,435,536 as a suitable reactant for introducing trialkoxysilyl groups into organic polymers containing ethylenically unsaturated hydrocarbon radicals.